Classical methods for determining the presence and number of bacteria in a sample are time consuming, tedious and labor intensive. Typically, a technician must prepare reagents and nutrients, mix the nutrients with agar, heat the mixture, pour the mixture into a petri dish, allow the agar to gel, obtain a test sample, dilute the test sample, add an aliquot of the diluted sample to the agar, incubate the inoculated plate for 24-48 hours and finally count the number of growing bacterial colonies in the petri dish. Products and processes which reduce the preparation time and which allow an earlier, rapid count of the bacteria would clearly be welcomed by those working in this field.
One example of a product which greatly simplifies the above preparation time is a dry culture device for growing microorganisms that is described in U.S. Pat. No. 4,565,783 to Hansen et al. In a typical device reported by Hansen et al., a cold-water soluble dry powder containing a gelling agent and microbial growth nutrients is coated on a waterproof substrate. A transparent, read-through cover sheet coated on a surface with an acrylate adhesive containing an indicating dye and powdered gelling agent are attached to the coated substrate.
When the device is used, a predetermined amount of an aqueous sample is typically placed in contact with the coated substrate and the cover sheet is placed over the sample and substrate. The aqueous sample hydrates the soluble dry powder which then forms a gelled medium capable of sustaining microbial growth. During the growth period, the indicator dye adhered to the cover sheet reacts in the presence of viable microorganisms to give a detectable response that allows visualization of bacterial colonies which are grown on the culture device. A dry culture device based on the report of Hansen et al. is commercially available as PETRIFILM plates (Catalog No. 6400, 3M, St. Paul, Minn.).
The dry culture devices of Hansen et al. are much simpler to use than conventional gelled agar medium/petri dish systems because there is no need for the user to heat and mix the growth medium, agar and other reagents and then add the mixture to petri dishes or pour plates. In addition, the devices of Hansen et al. are compact and easily disposed of and therefore are easier and safer to use.
In spite of the many advantages that the Hansen et al. devices have over conventional types of culture systems, the inoculated thin film plates must still be incubated for 24-48 hours before the number of bacteria may be determined. The ability to detect the presence or determine the number of bacteria at an earlier time may be very valuable in many circumstances. For example, earlier detection and rapid count of bacteria is important in the food industry. At the present time, the determination of bacteria only after an incubation time of 24-48 hours requires processors to delay distribution of food products and may allow the production of large amounts of contaminated products. Earlier detection of bacteria in food products would allow the processor to release food products for distribution at an earlier time period because contamination or lack of contamination could be established earlier. In addition, a processor would be able locate and correct a source of bacterial contamination without having to discard large amounts of contaminated products. Thus, detection of bacterial contamination in less than 24-48 hours would be extremely beneficial to food product producers.
Although the food industry would clearly benefit by determining bacterial contamination at an earlier time, other industries would also welcome the opportunity to detect bacteria more quickly. A need exists for products and processes which allow the early detection and rapid count of coliform bacteria.